The purpose of this research is to investigate the role of retinoids and specific retinoid-binding proteins in ocular tissues. Isolated, dark-adapted toad (Bufo marinus) retinas incubated in Ringers solution were used in studies to compare the effect of interphotoreceptor retinoid binding protein (IRBP) with bovine serum albumin (BSA) or Ringers solution alone on the release and protection of all-trans retinol from illuminated rod photoreceptors. Retinas were illuminated for 2 minutes (greater than 90% bleach), then incubated in darkness for a defined period (5-180 min). The generation of all-trans retinol in the retina was light dependent and all-trans retinol was the major retinoid in bleached retinas incubated in the presence of IRBP, BSA or Ringers alone. In preparations receiving IRBP the amount of all-trans retinol in the extracellular medium after longer incubations substantially exceeded (by 10 fold after a 3 hour incubation) that observed with BSA or Ringers alone. These results support the idea that in vivo, both the release of all-trans retinol from illuminated rods and all-trans retinols extracellular protection are promoted by IRBP, a known major component of the interphotoreceptor matrix. In other studies, the expression of retinoic acid receptors, RARalpha and RXRalpha in ARPE-19 human retinal pigment epithelial cells was examined. Northern blot analysis demonstrated the presence of both RARalpha and RXRalpha mRNAs in these cells. The nuclear extracts from ARPE-19 cells decreased the mobility of the RARE and RXRE oligonucleotides in gel shift assays, and cells treated with all-trans retinoic acid showed a marked increase in RARE binding activity. Therefore, the ARPE-19 human retinal pigment epithelial cell line has the potential to serve as a model system to study the effects of retinoic acid on gene expression in the retinal pigment epithelium. We have previously cloned and characterized a retinoid- and fatty acid-binding glycoprotein (RFABG), a member of the proapolipophorin gene family that is expressed in the Semper (cone) cells of Drosophila melanogaster eyes. We have now characterized the heme-binding properties of RFABG. Spectral analysis of purified RFABG revealed an absolute absorbance peak at 405 nm, which is typical for a heme-containing protein. Upon saturation of the protein solution with carbon monoxide followed by dithionite reduction, a red shift of the Soret peak to 424 nm and the characteristic alpha and beta bands at 567 and 539 nm were observed. Heme added to a solution of purified RFABG bound in a saturable manner with an affinity of 3.8x10-7 M. Thus, RFABG is also a heme-binding protein. - retinoids,interphotoreceptor retinoid-binding protein, retina, retinal pigment epithelium, retinoic acid receptors, drosophila